This invention relates to fluid dynamic bearings (also commonly referred to as “hydrodynamic bearings”) utilized in spindle motors for storage disk drive devices. The invention also relates to the method of manufacture of these fluid dynamic bearings. More particularly, the invention relates to a technological improvement of dynamic pressure characteristics of fluid dynamic bearings made of steel or stainless steel. Additionally, the invention relates to significant improvements in the configuration of bearing surfaces of accurate and high precision fluid dynamic bearings. Furthermore, this invention relates to spindle motors and storage disk drive devices utilizing improved fluid dynamic bearings.
FIG. 17 shows an example of a prior art fluid dynamic bearing. The fluid dynamic pressure bearing device shown in this drawing has rotating shaft 2 rotatably supported within a cylindrical through-hole of bearing sleeve 1. The bottom opening of the cylindrical through-hole of sleeve 1 is enclosed by counter-plate 3 thus enclosing the rotating shaft inside the bearing sleeve. A disc-shaped thrust plate 4 is affixed at the bottom end of the rotating shaft 2. Multiple arcuate spiral-shaped or herringbone-shaped dynamic pressure grooves (not shown in the drawing), are formed on the lower surface of thrust plate 4 opposite to counter-plate 3. Also, multiple dynamic pressure grooves (not shown in the drawing), are similarly formed on the lower surface of the step formed within the bearing sleeve above the thrust plate. Further, multiple spiral-shaped or herringbone-shaped dynamic pressure grooves 18 are formed on the inner circumferential surface of bearing sleeve 1. The bearing gap formed between the fixed bearing sleeve with the counter-plate and the rotating shaft with the thrust plate is filled with lubricating oil.
In recent years, there have been an upsurge in requirements for miniaturization, reduction in weight, and thinner profiles of hard disk drives, which are typical disk drive storage devices. At the same time, as the storage capacity of hard disks has increased, there was a growing demand for increased surface density. Therefore, a significant amount of research has been done on widely-used fluid dynamic bearings that resulted in a significant increase in utilization of fluid dynamic bearings for spindle motors in hard disk drives. The research concentrated in the area of forming more accurate and highly-precise dynamic pressure grooves as a way to accurately and efficiently generate dynamic pressure.
Electrochemical machining is currently known as a method for forming dynamic pressure grooves. However, when this method is used on a bearing sleeve made of sulphur free-cutting alloy steel having good machining properties, it is difficult to use electrochemical machining to dissolve and form multiple dynamic pressure grooves. It is also difficult to obtain the desired degree of accuracy and high-precision in the shape of ridges remaining between the dynamic pressure grooves. FIG. 3A shows that, when the surface of each dynamic pressure groove is viewed in cross-section, corner portion 24 connecting top portion 22 and sloped wall 23 of each ridge is rounded. This is the result of the disparity between dissolution characteristics of the sulfide inclusions comprising the free-cutting components, which are contained in the sulphur free-cutting alloy steel, and dissolution characteristics of the Fe, which is the principle component of the metal, during the electrochemical machining process. Additionally, the rounding of the corner portion is the result of dislodging and cracking of the sulfide inclusions exposed on the electrochemically machined surface.
It is important to optimize dynamic pressure efficiency (dynamic pressure/axial torque loss), in order to create dynamic pressure more accurately and efficiently. In FIG. 3B, it is preferable that the ridge-groove ratio is as follows:
                                                        dynamic              ⁢                                                          ⁢              pressure              ⁢                                                          ⁢              groove              ⁢                                                          ⁢              width                        ,            Bv                                              width              ⁢                                                          ⁢              of              ⁢                                                          ⁢              top              ⁢                                                          ⁢              portion                        ,                          B              ⁢                                                          ⁢              1                                      =                  1.0          ∼          1.3                                    (        1        )            Additionally, it is also preferable that the groove depth ratio is as follows:
                                                                                                              distance                    ⁢                                                                                  ⁢                    from                    ⁢                                                                                  ⁢                    the                    ⁢                                                                                  ⁢                    surface                    ⁢                                                                                  ⁢                    opposite                    ⁢                                                                                  ⁢                    the                    ⁢                                                                                  ⁢                    dynamic                                    ⁢                                                                                                                                                                                  pressure                    ⁢                                                                                  ⁢                    groove                    ⁢                                                                                  ⁢                    to                    ⁢                                                                                  ⁢                    the                    ⁢                                                                                  ⁢                    bottom                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    the                    ⁢                                                                                  ⁢                    groove                                    ,                                      h                    ⁢                                                                                  ⁢                    0                                                                                                                          depth                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                groove                            ,              hg                        ⁢                                                                =                  2.1          ∼          2.3                                    (        2        )            It is difficult, however, to achieve (1) and (2) in mass production due to the reasons outlined above.
There are other problems caused by insolubility and dislodging of sulfide inclusions. Surfaces that are finished by milling and surfaces finished by electrochemical machining have problems of irregular surfaces due to dislodging and insolubility, thus aggravating surface roughness. Additional problems are caused when sulfide inclusions, which are exposed on the machined surface or dislodged from the surface, build up, and become lodged in between the shaft body and the bearing part.
As a result of these problems, bearing rigidity in fluid dynamic bearings declines and axial torque loss increases so that the rotational precision and service life of spindle motors for storage disk drive devices declines, power consumption and starting times increase, leading to significant problems for storage disk drive devices, especially given the trend toward miniaturization and a slimmer profile.
Methods disclosed in, for example, Japanese Laid-Open Patent Publication 2001-298899 and Japanese Laid-Open Patent Publication 2002-119584 have been utilized to prevent dislodging of sulfide inclusions. The above references propose the removal of sulfide inclusions using acids and alkalis after machining the parts. However, these removal methods present an impediment to the reduction of manufacturing costs by adding complexity to the manufacturing process. Handling of acids and alkalis is hazardous and represents a danger to the environment. Additionally, the disclosed methods offer no solution for improvement in the accuracy and high-precision of the surface configuration of fluid dynamic bearings. Moreover, the use of acids and alkalis to remove sulfide inclusions ends up dissolving the matrix as well, and has the defect of rounding the corner portions of the ridges as well.